^For the most part, the stories from Ys games are self-contained, with a few characters/references carrying over to later entries (nothing too significant, though). The only games that are closely linked are the first two titles, and they tend to be bundled together anyway, so that's no big deal, there.
Anyway, I do suppose it's a bit early to read too deeply into Adol's apparent absence.... though I do wonder if it's possible if Falcom will pull an Origin and focus on a completely new cast instead? Could be interesting, I suppose....